dillon1000/react

Use sequence terminal, fix most remaining order-of-evaluation bugs

Changes the lowering for sequence expressions to use the new terminal. When 
converting to a ReactiveFunction, we convert these terminals into 
ReactiveSequenceValues, which nests the instructions and preserves order of 
evaluation in the output. 

The only catch is constant propagation — constant propagation breaks 
order-of-evaluation because it can effectively copy the final value of a 
sequence elsewhere, leaving the original sequence in the wrong place. I'll 
address that in a follow-up.